Single Parity implemented as RAID4 (effictively equivalent to RAID5) as WAFL does not need to distribute Parity
and Dual Parity implemented as RAID-DP which complies with the RAID6 definition.
On any aggregate you can change from RAID-DP to RAID4 with a single instruction or click. The second parity disk becomes free and will be reused as a spare after zeroing, you can also add it into the aggregate as a data disk. Be careful - once added you cannot take it out again!!
You can also go from RAID4 to RAID-DP if you have enough spares (may need to check raidsetsize and maximums). This will take time as it builds the parity information.
In your environment simply go into the GUI and change you aggragate from RAID-DP to RAID4 (you will of course decrease protection), but in very small configurations this is often a satisfactory solution.
For your purposes, RAID 4 = RAID 5 (i.e. ability to withstand a single disk failure).
And as noted, it's easy to change back and forth...just disk intensive so good to do after hours (I haven't changed from RAID4 to RAID DP or vice versa recently but have advised customers about it without their having any problems (easy enough they just did it themselves)).